1. Field of the Invention
The present invention relates to a method for steering a payload beam of a satellite towards an intended service region in a manner which improves payload beam coverage over the intended service area in the presence of non-geostationary orbit effects while retaining simple steering logic.
2. Background of the Invention
Payload performance for many spacecraft, such as communication satellites, is often enhanced if there is no relative motion between the payload beam and the intended service region. If the intended service region is a fixed region on the ground, this can be achieved if the satellite position remains fixed with respect to the revolving earth.
This can be accomplished by placing the satellite in a "geostationary"orbit. The benefits of geostationary orbits are such that there are scores of satellites currently in geostationary orbits, and the techniques for operating satellites in geostationary orbit are well known in the art.
A geostationary orbit is a circular orbit that lies in the plane of the Equator, with an orbit period equal to the Earth's rotation period, and an easterly orbit velocity. In technical terms, a geostationary orbit is a prograde orbit with zero eccentricity, zero inclination, a period of one sidereal day (approximately 86,164.0907 seconds) and a semimajor axis of 42164.172 km.
Ideally, a satellite in a geostationary orbit stays directly above a point with fixed longitude and zero latitude (the equator). Practically, satellites are termed geostationary satellites if they remain substantially geostationary. The allowable deviation depends on context, but commercial "geostationary" communications satellites are typically designed to stay within 0.1 degrees in latitude and longitude of their assigned station.
The price of staying in geostationary orbit is that a satellite in geostationary orbit is constantly being pulled away from that orbit by environmental forces that must be countered if the satellite is to remain in geostationary orbit. Gravitational accelerations due to the moon, the sun, and the nonuniform earth all work to introduce orbit inclination, drift, and eccentricity. Geostationary satellites must therefore allocate much of their launch weight to propellant rather than payload, and when that propellant is exhausted, the benefits of geostationary orbit no longer obtain. The largest amount of propellant must be expended to resist the tendency of the sun and moon to increase the orbit inclination.
These costs are well known, and so many satellites are operated in geosynchronous orbits rather than geostationary orbits. Broadly, a geosynchronous orbit is an orbit that has the same period as the earth's rotation, but may have nonzero eccentricity and inclination. By this definition, a geostationary orbit is a geosynchronous orbit, but the term "geosynchronous" is sometimes used to distinguish from "geostationary". In this work, the tern "geosynchronous" shall be used to refer to orbits whose period is substantially that of the rotation of the earth, but which exhibit variations of more than 0.25 deg in latitude or longitude.
The consequences of geosynchronous operation is relative motion of the satellite relative to earth-fixed points. For points in the service region, the azimuth, elevation, range, range rate, and angle above the horizon of the satellite now vary. These have consequences on payload performance. For example, for communications satellites, these effects cause ground antenna gain losses due to azimuth and elevation variation, variations in equivalent isotropic received power (EIRP) due to range variation, frequency variations due to Doppler effects of range rate, and variations in EIRP due to atmospheric attenuation variations with horizon angle.
Due to the payload consequences for non-geostationary satellites, there is interest in keeping the orbit close to geostationary, and many geosynchronous satellites are kept within a latitude range of plus or minus six degrees in service, and within a longitude range of plus or minus 0.1 degrees at the equatorial plane crossing. Such satellites have been variously termed "nearly geostationary", "quasi-stationary", "passively controlled geosynchronous", "low inclination geosynchronous", "slightly inclined" or "inclined geosynchronous" orbits. Typically, the orbit period, eccentricity and true anomaly are controlled to some degree by "east-west station keeping" or "drift and eccentricity" maneuvers, but the orbit inclination control is limited or non-existent. This type of control is sometimes called "free inclination drift", or a "free-drift strategy".
For non-geostationary satellites, the apparent angle between points in the intended service region and attitude determination references (such as a beacon site located in the service region, the earth center, the sun or the fixed stars) now varies from the same angle seen by a satellite in geostationary orbit. This will cause variations in pointing of the payload beam relative to the service region unless accounted for by some form of attitude control steering of the payload beam. Such pointing variations can cause the payload beam to deviate sufficiently from the intended service region that coverage of the intended service region is insufficient, or unacceptable intrusion and interference in other regions occurs.
If attitude steering is not provided for the effects of the non-geostationary orbit, the pointing variations are typically least when the attitude determination reference is a beacon site in the service region, and greatest when the attitude determination reference is the sun or stars. This is because the parallax effects between points in the service region and an attitude reference point are, all other things being equal, larger the further the point in the service region is from the attitude reference point. Note that, since the effects of parallax will be different for different points in the service region, attitude steering of the payload beam can reduce, but cannot totally eliminate the pointing variations over the entire service region. If the residual effects are to be reduced further, techniques such as reshaping the payload beam should be considered.
The desirability of correcting the attitude of the payload beam for non-geostationary orbit effects is well known, and many techniques have been developed for such correction.
One technique is Beacon-Target pointing, which points the satellite at a radio-frequency beacon in the intended service region. While this is a useful technique, by reason of the reduced parallax discussed above boresight pointing at a surface point will not correct the adverse effect of an inclined orbit as well as pointing at a subterranean point, as will be demonstrated later. The effectiveness of this approach is further hampered by the economic, political and physical constraints that may preclude placing a radio beacon at the geographically ideal surface location. Moreover, the ground beacon approach suffers from the added costs and constraints of building and maintaining a ground station, and is vulnerable to periodic beacon outages or failures.
Another approach is to use what may be characterized as Generalized Offset Function pointing. Many forms of offset functions have been employed, such as sinusoidal correction (sinusoidal earth sensor roll offset and yaw momentum commands, updated for inclination changes), Fourier series, on board correction tables (daily consultation of a 128 point table for offset of pitch, roll and yaw momentum, relying on the earth's infrared horizon as a pointing reference), as well as other methods. A more detailed explanation can be found in U.S. Pat. No. 5,100,084. These approaches typically require an extensive amount of ground interaction to keep the steering method up to date, and also require a great deal of on board processing which taxes the on board hardware and software systems of the spacecraft. Another well established method is Ephemeris based pointing. Ephemeris based pointing has been used to keep the payload boresight pointed at a desired surface target point despite non-geostationary orbit effects. This technique and use is well-known (e.g., the Redisch and Hall paper from EASCON '74, and the Lebsock paper AIAA 88-4308-CP). In Ephemeris based pointing, the commanded payload attitude is calculated by knowledge of the satellite position and the position of the target point. From this information, the desired payload beam attitude is calculated and the satellite attitude control system implements it. The satellite position knowledge is typically computed by an on-board model of the orbit, and the model parameters are typically updated periodically from the ground. In the examples above, the target point was a surface point, so the commanded attitude profile was the same as would be produced by Beacon-Target pointing. As noted above, this attitude profile is generally not as good as that produced by a subterranean target point. The deficiencies of this approach were recognized in the Loh paper AIAA 92-1940-CP, which proposed to improve the compensation of orbit inclination effects by averaging the commanded attitudes obtained by pointing at two or more surface points. Such an approach may experience less pointing error than approaches using a single target point, but unfortunately involves more intensive computations, thus taxing on board hardware and software systems. Such an approach is also more complicated to reprogram from the ground or communicate to operators or designers than a single point target method.
Consequently, there exists a need for an improved strategy for steering satellites in non-geostationary orbits which improves pointing performance and ground coverage and which is less taxing to the on board hardware and software systems. Ideally, such an improved steering strategy should be less computationally intensive and easier to re-program from the ground than existing steering strategies.